#app4 {
}
@keyframes change {
    0% {
        background: red;
    }
    100% {
        background: blue;
    }
}
#app4 .circle {
    width: 30vw;
    height: 30vw;
    border-radius: 50%;
    border: 1px solid red;
}
#app4 .circle.active{
    animation: change 1s infinite alternate linear;
}